Lighting using solid-state devices such as LEDs is gaining momentum. The use of LEDs for lighting has several advantages over the use of conventional light sources, including high efficacy (lumen per watt), small form factor and durability. It is reported that the average lifetime of an LED is approximately 50000 hours instead of 2000 hours of an incandescent light bulb. As a result, LEDs are the preferred light source in difficult-to-replace lighting fixtures (street lights, traffic signal lights) and/or in fixtures that require higher reliability (automotive light) for safety reasons.
Despite the fact that LEDs have a long life time, like many other light sources, the light output from a LED decays over time (by an aging process). This ultimately leads to LED failure. It has been suggested that an LED with a light output level at 70% of its pristine value needs to be replaced.
Depending on the operating conditions of the LED, the time that it needs to be replaced varies greatly. Currently, light sources including LEDs are replaced according to a fixed schedule. In this manner, some of the lights are still operational and are discarded. It can also happen that a light source is burnt out before its next scheduled maintenance. Evidently maintenance according to a fixed schedule is not the best solution for lighting fixtures especially for those at difficult to reach places and if safety (or cost) is of concern.
Knowing the LED light output flux in comparison with its pristine value is desirable for making the maintenance schedule for LED fixtures, and for controlling the LED light output as well as colour (in the case of a cluster of different colour LEDs).
LED light output is conventionally monitored and measured by an external optical sensor such as a photodiode. This method is robust but has a few disadvantages. First of all, there is a need for an external sensor and wirings to this sensor. This adds to the overall cost of the system. Secondly, if an LED lighting system consists of more than one LED, multiple sensors or a time sharing mechanism between the sensor and the LEDs needs to be in place. In the case of multiple LEDs sharing one sensor, the performance requirements on the sensor is usually very high in term of speed, accuracy and the dynamic range.
In addition, the signal processing circuitry connected to the sensor must also be of high performance and therefore of higher cost. Using an external optical sensor to quantify LED flux output can be rather accurate under ideal lighting conditions i.e. without interference from other light sources. However, inaccuracy can arise in non-ideal lighting conditions. Finally, deterioration of the optical sensor and of the light path between the sensor and the LED can also contribute to the inaccuracy of the method.
There is therefore a need for a method to estimate the output flux of a LED without needing the use of an optical sensor. Most of the issues with existing LED light output monitoring/measuring methods as described above could then be overcome.